1. Technical Field
The present invention relates to a fuel cell power generation system.
2. Description of the Related Art
A fuel cell power generation system produces electricity by electrochemically reacting both a fuel gas, such as methanol, including a large amount of hydrogen and air extracted from the atmosphere as an oxidizing agent at a gas diffusion electrode. Such an environmentally friendly fuel cell power generation system may serve as a clean energy source for solving global environmental problems caused by the use of fossil energy and the increased power demand.
The fuel cell power generation system combines a stack, on which a plurality of unit batteries for producing electricity are stacked, and a hydrogen tank, which supplies hydrogen as fuel to the stack.
The hydrogen tank is a part of a fuel supplying device that stores hydrogen in order to supply the hydrogen to the stack. Some methods of storing hydrogen include storing hydrogen through the use of a pressurized tank, storing liquid hydrogen at an extremely low temperature, storing hydrogen in a porous material such as a carbon nanotube (CNT) and storing hydrogen by using metal hydride, such as metal powder, having hydrogen absorption characteristics.
The method of storing hydrogen through the use of a pressurized tank stores hydrogen as a compressed gas at the pressure of at least 300 bar and thus requires a large volume and long-term stability, making it difficult to be employed in a small device such as a mobile device. Likewise, the technology of storing hydrogen as liquid hydrogen at an extremely low temperature also requires an additional device and a large volume, making it difficult to be employed in a mobile electronic device.
The method of storing hydrogen in a porous material such as a carbon nanotube (CNT) has an advantage of being simply implemented in small scale. However, there are practical difficulties in application since the relatively high pressure of over 100 bar is required and the amount of hydrogen being absorbed is too small for practical use.
A notable feature of the metal hydride is its ability to reversibly absorb and/or desorb hydrogen at a low temperature and pressure. The metal hydride has been of interest recently because of its potential as a hydrogen storage material for mobile devices. Therefore, a hydrogen tank using the metal hydrides is currently in the research and development phase.
FIG. 1 is a graph illustrating operating temperatures and pressures of various types of metal hydride, such as NaAlH4, LaNi5H5, MgH2, Mg2NiH4, LaNi4H5 and Li-Amide. At such operating temperatures and pressures, hydrogen can be absorbed into a space between metal hydride grids and desorbed from the space between the metal hydride grids when the operating pressure is lowered.
Among the kinds of metal hydride described above, if hydrogen can be absorbed and/or desorbed at room temperature, which is between 25 and 100 degrees Celsius, and room pressure, which is between 1 and 10 atm (the shaded box), the metal hydride may be appropriate for the manufacture of the fuel cell power generation system using the hydrogen tank for use in mobile devices because of the ability to operate without an external heat source or high-pressure hydrogen recharging. Nevertheless, when hydrogen is desorbed, the operating temperature is decreased unless hest is continuously supplied, and thus some hydrogen stored in the storage tank may not be used.